1. Field of the Invention
The present invention relates to a solder ball mounting apparatus, and method, for mounting solder balls on pads of a substrate.
2. Description of the Related Art
In a ball grid array which is one of a surface mounting type semiconductor device, solder bumps are formed on pads located on a substrate. As for a method for forming the solder bumps, ball-shaped solder lumps were made separately. Flux was applied to pads of the substrate, and the solder balls were placed thereon. The solder balls were fused again. In this way, the solder bumps were formed. For mounting a large number of solder balls on pads arranged on the substrate at narrow pitches, there are various problems such as positioning of the solder balls on the pads and a method for holding solder balls. For solving the problems, a large number of improvements have been proposed.
For example, one of the methods is disclosed in Japanese Patent Application Laid-Open NO. Hei-07-283521. According to this method, a holding mechanism having a plurality of partitions is electrified. Solder balls are lifted by electrostatic force. The solder balls are placed in the partitions and held therein. The holding mechanism is moved so as to be located over the substrate. Thereafter, supply of static electricity is stopped. The solder balls are separated from partitions by ion plow. As a result, solder balls are mounted on the pads.
In the above described mounting method, however, there is a fear that the holding mechanism is charged with more static electricity than needed and excessive solder balls stick to the holding mechanism. For example, a solder ball is attracted by a partition portion, and the solder ball sticks to the holding mechanism in such a state that the solder ball is in contact with an adjacent solder ball. Even if static electricity is removed by subsequent ion plow, and ion flow is conducted at the time of mounting on the substrate, it is difficult to remove static electricity uniformly from the entire surface of the holding mechanism. Nonuniformity of the static electricity removing state and delay of falling of solder balls resulting therefrom are caused. This results in a problem of displacement of solder ball mounting positions. In any case, it is difficult to control solder ball holding and mounting by using a mechanism for generating static electricity.
Furthermore, a method for forming solder bumps is disclosed in Japanese Patent Application Laid-Open NO. Hei-02-278831. According to this method, a hole is formed in a predetermined position of a plate which is not wettable by fused solder. A solder member is held by this hole. The plate is put upon an insulative substrate so that the solder member may be opposed to a conductor for solder bump disposed on the insulative substrate. The solder member is heated, and the solder member is fused with the conductor.
Furthermore, a semiconductor chip mounting method is disclosed in Japanese Patent Application No. 2551370. According to this method, conductive paste is stuck to pads formed on a substrate by screen printing. Thereafter, metallic balls are mounted on the conductive paste.
Furthermore, a method for fabricating soldered electronic parts is proposed in Japanese Patent Application Laid-Open NO. Hei-09-8172. According to this method, a solder ball is picked up by using vacuum suction, and flux is applied to the solder ball. Thereafter, a suction head is moved so as be located over a substrate, and the solder ball is mounted on an electrode of the substrate.
However, these methods disclosed in Japanese Patent Application Laid-Opens and Japanese Patent Application have the same drawbacks as those of the above described conventional technique.